In order to minimize weight and to meet stringent safety requirements, in motor-vehicle construction it is standard to make more and more of the vehicle frame parts or elements of and high-tensile-strength steel. Such elements, in unibody construction, include side impact beams, A- and B-columns, bumpers, side rails, and cross rails, that are increasingly being made from a hot-shaped and pressure-hardened or high-strength steel.
GB 1,490,535 of describes a method for pressure-forming and hardening a relatively thin steel sheet having good dimensional stability in which a sheet made of boron-alloyed steel is heated to above the AC3 temperature and then in less than 5 seconds is pressed into the final shape between two indirectly cooled tools with a significant change in shape. After the hot-shaping the workpiece stays in the press for to rapid cooling such that a martensitic and/or bainitic structure is obtained. This way, a product is obtained that has high shape accuracy, good dimensional stability, and high strength, and that is highly suitable for structural and safety elements in motor-vehicle manufacture. This process is referred to in the following as heat-forming and pressure-hardening.
US 2005/0199433 based on application Ser. No. 10/962,982 of Danger describes the hot-shaping and pressure-hardening of a type of steel having substantially the following composition in weight percent:
carbon0.18 to 0.3%silicon0.1 to 0.7%manganese1.0 to 2.5%phosphorusmax. 0.025%chromiumup to 0.8%molybdenumup to 0.5%sulfurmax. 0.01%titanium0.02 to 0.05%boron0.0015 to 0.005%aluminum0.01 to 0.06%balance iron and usual smelting-related impurities. After hot-shaping and hardening, this steel has a yield point Rp0.2≧950 N/mm2, tensile strength Rm≧1350 N/mm2, and elongation A5≧8%. This product is marketed under the trade name BTR 165. A hot-shaped and hardened structural and/or safety component made of BTR 165 is then clamped in a heat chamber and coated on all sides with a sherardizing powder made of zinc at less than 320° C. The hot-shaped and pressure-hardened structural and/or safety component must be heated as little as possible during the solid diffusion process so that the strength of the hardened steel is not significantly affected. Thus the piece is subsequently worked at temperatures below 320° C.
U.S. Pat. No. 6,758,921 describes is a method for producing a bending-resistant torsionally yielding tube for use as a transverse support for a torsion-bar rear axle in which a tube made of tempered steel is first made by U-shaped cold forming with a torsionally yielding central longitudinal section while ensuring torsion-proof end sections, the thus formed tubular profile subsequently being annealed at least in partial sections at a temperature between 920° C. and 960° C., subsequently hardened in water at above the AC3 point, subsequently tempered at a temperature between 240° C. and 320° C. over a period of about 20 minutes, then subjected to at least one outer surface hardening, and finally subjected to further shaping process to make a torsion-bar rear axle.
U.S. Pat. No. 5,972,134 of Buschsieweke describes homogeneously heating a workpiece of BTR 165 initially to a temperature between 900° C. and 950° C., then shaping the workpiece into a shaped part in a press, and then tempering the shaped part in the press in order subsequently to bring some areas of the shaped part to a temperature between 600° C. and 900° C. in less than 30 seconds. Areas with higher ductility are created in the workpiece in this manner. At temperatures between 600° C. and 900° C. there is a major structural change in the steel, which means that the mechanical values change to those of unhardened steel. Therefore the steel no longer has high tensile strength in the ductile areas.
Structural and safety elements are for instance side impact beams, bumpers, B-columns, and side rails. A partially hardened B-column made of BTR 165 is disclosed for instance in U.S. Pat. No. 6,524,404 of Gehringhoff. As a rule, B-columns, bumpers, and side impact beams are loaded to bending in a collision. This is not the case for instance for a side rail. Side rails run in the vehicle's longitudinal direction and are also produced from high-strength steel. If the vehicle is struck from the front or rear, crash energy is conducted into the side rail in the longitudinal direction. The side rail should have high tensile strength, but starting with a defined load should convert crash energy to deformation by crumpling. However, the described BTR 165 material when hardened for instance is not ductile enough to crumple. As a rule, the hardened material does not deform when loaded so that at times there can be brittle element ruptures. This is disadvantageous for decreasing the energy.
Finally, EP 0,753,595 of Bergs describes tubes for making stiffeners or stabilizers that prevent frame deformation. They are made of a specific boron-alloyed steel but still have some failings in that giving them a targeted deformation response is very difficult.